A typical speaker, for example, comprising a dome diaphragm has a voice coil wound on a voice coil bobbin connected to the diaphragm. The voice coil is disposed in a magnetic gap of a magnetic circuit so as to vibrate therein, and the diaphragm is fixed to a speaker frame through an edge. In the speaker structured as described above, upon receipt of an electric signal at the voice coil from the outside, the electromagnetic action produces a drive force on the voice coil, so that the diaphragm vibrates to emit a sound wave.
In the speaker with the above structure, the space inside the voice coil and the diaphragm is substantially hermetically sealed. Because of this, disadvantageous problems may arise: during the operation of the speaker, the vibration of the diaphragm causes an air temperature in the sealed space to become relatively high; the heat generated on the voice coil cannot be easily dissipated; the air temperature in the sealed space, for example in a vehicle-mounted speaker, rises to become relatively high when the temperature in the car's interior is relatively high at midsummer or the like; the above-described heat or back pressure causes deformation of the diaphragm; the deformation of the diaphragm reduces the sound quality of the reproduced sound; and the like.
A cone speaker device disclosed in Patent Document 1 has a cap provided on a bobbin on which a voice coil is wound, and a through hole drilled in the central portion of a center pole of the outer-magnet-type magnetic circuit inserted into the bobbin. The high temperature air inside the bobbin is dissipated through the through hole to the outside.
A dome speaker disclosed in Patent Document 2 has a very-small-diameter air vent formed through a diaphragm or an edge for adjustment of the back pressure of the diaphragm.
Patent Document 1: Japanese Patent application Laid-Open No. 2002-271889
Patent Document 2: Japanese Patent application Laid-Open No. 2002-247687